Aluminum nitride has been attracting attention as a packaging material or a substrate material best adapted for heat dissipation because of its high thermal conductivity, excellent electrical characteristics, such as insulation resistance, dielectric strength, and dielectric constant, and excellent mechanical characteristics, such as strength.
Production of an aluminum nitride sintered body having excellent thermal conductivity requires molding of an aluminum nitride powder followed by densely sintering.
Since aluminum nitride by itself is hardly sintered under atmospheric pressure, pressureless sintering of aluminum nitride has conventionally be carried out with the aid of an oxide, such as CaO and Y.sub.2 O.sub.3, taking advantage of the reaction between such a sintering aid and Al.sub.2 O.sub.3 present on the aluminum nitride skin layer to achieve denseness.
For example, JP-B-47-18655 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses addition of Y.sub.2 O.sub.3 for obtaining a dense sintered body, and JP-B-58-49510 proposes addition of CaO, BaO, SrO, etc. to obtain a dense aluminum nitride sintered body having a relative density of 98.5% or more.
In the case of adding Y.sub.2 O.sub.3 for obtaining a dense sintered body having high thermal conductivity by pressureless sintering, a high temperature of 1800.degree. C. or even more should be required. Also in the case of adding CaO, pressureless sintering is usually carried out at a high temperature of 1700.degree. C. or more.
Alumina which is currently employed as an IC packaging material or a substrate material is usually sintered at a temperature of from 1500.degree. to 1600.degree. C. to obtain denseness. Compared with this temperature, the sintering temperature required for obtaining an aluminum nitride sintered body with the aid of the above-described sintering aid is considerably higher, and this has been a bar to reduction of cost for production of an aluminum nitride sintered body.
In this connection, it has been proposed to add an oxide or fluoride of a rare earth metal and an oxide or fluoride of an alkaline earth metal to make it feasible to densify at 1600.degree. C. as disclosed in JP-A-61-209959 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, the thermal conductivity attained by this process is about 100 W/mk at the highest.